1. Field of the Invention
The invention relates to a method for fabricating semiconductor device, and more particularly to a method of using polysilicon as stop layer in a replacement metal gate (RMG) process.
2. Description of the Prior Art
In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
However, in current fabrication of high-k metal gate transistor, voids are often formed during the deposition of work function metal layer for fabricating multi-VT devices and affect the performance of the device substantially. Hence, how to resolve this issue has become an important task in this field.